Going a step further, a team of researchers from IISc Bangalore has improved upon the water purification membrane they developed in 2014. The membrane allows higher flow of water across it and kills nearly 99 per cent of E. coli present in water. The results of the study were published recently in the journal Nanoscale.
Instead of creating a membrane with sub-micron pore size, a team led by Prof. Suryasarathi Bose, the corresponding author of the paper from the Department of Materials Engineering, IISc produced a more permeable structure by creating pores that are bigger in size and more interconnected.
Bigger and more tortuous pores were produced by mixing equal amounts of two polymers — polyethylene (PE) and polyethylene oxide (PEO). Since PEO is soluble in water unlike PE, pores tend to form when the membrane containing PEO is dipped in water. Earlier, the researchers used tiny amount of PEO and sheared it at high speed to produce tiny droplets of PEO to create smaller pores.
“We took equal amounts of PE and PEO so we get more tortuous pores upon removal of PEO. This is not possible if we take tiny amounts of PEO,” says Prof. Bose.
Besides being tortuous, the pores are also asymmetrical — the pore dimensions are not uniform throughout. At some places the pores get so narrow that they tend to be as small as the micro holes that the team produced two years ago. Explaining the logic behind having asymmetrical pores, Prof. Bose says: “If the pores are asymmetrical then bacteria and other contaminants will have a tougher path to pass through, so they will get trapped.”
The pores are also well connected thereby increasing the ability of water to pass through the membrane.
When two polymers are mixed and subjected to post processing application like hot pressing the initial PEO droplets tend to become bigger. The bigger droplets of PEO tend to leave bigger pores. “To prevent this and control the morphology we added maleated polyethylene. The maleated polyethylene does not allow the droplets to get bigger,” Prof. Bose says. “Maleated polyethylene basically interacts with PE (polyethylene) on the one hand and reacts with PEO on the other hand. So it is a kind of interfacial stabilising agent and doesn’t allow the morphology to coarsen.”
In their earlier attempt, to render the membrane antibacterial, grapheme oxide was mixed with the two polymers and the graphene oxide was made functional with amine groups. “Earlier the antibacterial effect was not significant as graphene oxide was embedded inside the membrane. But now we have made it more effective by anchoring graphene oxide on the surface of the membrane,” he says.
Antibacterial studies though direct contact of E. coli with graphene oxide resulted in 100-fold reduction in E. coli colony forming units at the end of 4 hours of contact with the membrane. According to him, graphene oxide has very sharp edges and this helps in piercing and destroying the bacterial cell wall. Also, the amine group of graphene interacts with the phosphate group of the lipids present in the cell and generates reactive oxygen species that eventually destroys the cell membrane.
Since polyethylene is inert, the researchers had render suitable surface modification to anchor graphene oxide on to it, which otherwise would have been very difficult.
Lab studies have revealed that there is unimpeded permeation of water across the membrane suggesting that anchoring the graphene oxide on the surface does not clog the pores.